I made an air gun from a CO2 fire extinguisher once
It was a fairly simple affair a length of aluminium tubing jammed in the funnel and gaffer taped in place. You would muzzle load it with D-Cell batteries wrapped with a bit more gaffer tape to make them a snug fit.

The first incarnation had a 6' barrel and needed 2 people to operate, one to load and aim and one to pull the trigger, I subsequently took a hack saw to the barrel and reduced it to about 3' making it possible to fire single handed.

It didn't have huge power, about a 25-30 yard effective range and fairly accurate but it was great fun due to the size of the projectiles and it had terrific intimidation value. I used to keep it under my desk at one point for fear of dodgy customers.
( CQ Knows the truth, all of it., Thu 17 Nov 2011, 13:49, Reply)